The problem addressed by the invention is how to provide terminal overspeed protection for a high speed elevator.
Elevators are presently provided with a plurality of bracing devices which are designed for use in normal operation of the elevator, as for example to hold the elevator car in place where it stops at a landing, and which are designed for use in emergency situations such as stopping the elevator car from plunging into the hoistway pit.
One bracing device for downward motion is by means of governor triggered safeties. A governor rope is looped over a governor sheave at the top of the hoistway and a tension sheave at the bottom of the hoistway and is attached to the elevator car. When the governor rope exceeds the rated speed of the elevator car by a limit, the governor grabs the governor rope, pulling up two rods connected to the elevator car, thereby pulling up two wedge safeties which pinch the guide rail on which the elevator car is riding. This braces the elevator car.
A second braking device for elevator downward motion is a buffer. Buffers are devices which are designed to stop a descending elevator car that moves downwardly beyond its normal limits of travel. Elevator pit buffers are commonly spring buffers or oil buffers, the former being typically used for elevator speeds of up to 200 feet/min. and the latter for speeds above 200 feet/min.
It becomes more difficult to decelerate the elevator car by means of a buffer as elevator speed increases; ultra high-speed elevators (above 1,800 feet/min.), which are highly desirable in high-rise buildings, require excessively long buffer pistons in order to provide adequate protection for passengers.
A reduced stroke buffer is shown in commonly owned U.S. Pat. application Ser. No. 07/914,822, "Pit Buffer Assembly for High-speed Elevators," filed on Jul. 19, 1992, which includes a crossbeam mounted on the elevator car guide rails by means of safety brakes which allow limited movement of the crossbeam when a downward force is exerted on it. The majority of the braking force for the descending elevator car is provided by the safety brakes on the buffer crossbeam.
For most high-speed elevator cars, oil buffers are used with a reduced stroke to give the elevator car an average retardation not exceeding 32.2 feet/sec..sup.2 when they are brought to rest after striking the buffer at 115% of the rated speed or a reduced speed if an emergency terminal speed limiting device is installed.
Even for the reduced stroke buffer, the minimum stroke required for an ultra high-speed elevator is 205 inches long, and a conventional oil buffer would require a total buffer length of approximately 40 feet.
The elevator codes usually allow reduced stroke buffers, but also require an emergency terminal stopping device which includes switches mounted on the elevator car in conjunction with governor overspeed contacts which turn off power to the motor driving the elevator car and apply the sheave brake when a cam contacts a vane. But if the emergency terminal stopping device (ETSD) fails, the elevator car could strike the buffer at the rated speed or faster and the desired average retardation of 32.2 feet/sec..sup.2 can be greatly exceeded.
The elevator brake devices described above are not operated in an instance where the elevator car is moving in the upward direction in the hoistway.
Commonly owned U.S. Pat. application Ser. No. 07/941,504, "Stopping of Elevators in the Up direction," filed Sep. 8, 1992, discloses a stopping plate provided in the overhead area in an elevator hoistway above the uppermost landing in the building. The stopping plate is mounted in the hoistway and is operable to stop upward movement of the elevator car without impacting the main components of the elevator car. There is provided a pair of inverted wedge safeties mounted on the stopping plate and guide rails. If the elevator car rises above the uppermost landing, the safeties will be set by the elevator car contacting the stopping plate, thereby limiting further upward motion of the elevator car.
It is desirable to have an elevator brake for arresting movement of the elevator car in the up direction and also desirable to arrest car movement in the down direction without a buffer.
Thus, the presently available solutions to the aforementioned problem involve long buffers or reduced stroke buffers in conjunction with long ETSD vanes.